Support hidden lines in languages other than Rust

[jannik4]

This is based on the work in #1339 by @thecodewarrior adapting the proposal by @dtolnay.

Changes:

• Moved the hidelines settings from output.html.playground to output.html.code, because it has nothing to do with the rust playground, but applies to all code blocks.

Example

# book.toml

[output.html.code.hidelines]
python = "~"

# using global settings

```python
~hidden()
nothidden():
~    hidden()
    ~hidden()
    nothidden()
```

# using local override

```python,hidelines=!!!
!!!hidden()
nothidden():
!!!    hidden()
    !!!hidden()
    nothidden()
```

---

(Since the changes (especially the examples in the mdbook) are based on your work @thecodewarrior I would like to add you as co-author if you want.)

[thecodewarrior]

This looks great! I don't know how co-authorship works, but that sounds good to me :D

[TheZoq2]

Sorry to leave a "any updates on this comment", but this would be very useful to me. Is there anything holding it back from being merged (once the new merge conflicts have been resolved)? Could I do anything to help?

[ehuss]

Thanks, I wanted to make a few small changes, but I couldn't push to the branch here, so I opened a new PR at #2093 to merge this.